OVERVIEW
Hydatid disease is a parasitic disease of humans caused by Echinococcus granulosus.
• It occurs world-wide and is endemic in sheep rearing areas including the Indian subcontinent.
• Echinococcus granulosus a cestode parasite and the adult worms inhabit the small intestine of dogs and other canines, which are the definitive hosts. Eggs are released in their feces and are ingested by intermediate hosts where the larvae are then released in the duodenum. Intermediate hosts can be sheep/goat or reindeer.
• Humans are accidental intermediate hosts. Contaminated water and vegetables are the main cause in human infestations.
• The liberated larvae travel in the blood stream and can encyst in liver, lungs and other organs
• Ultrasound and CT scan are the most important diagnostic tools to rule out complications and planning treatment.
• Modern treatment of hydatid cyst of the liver varies from surgical intervention to percutaneous drainage and medical management. Surgery is the preferred treatment and can be done by the open method or laparoscopic approach.

PATHOGENESIS/LIFE CYCLE OF HYDATID DISEASE
The life cycle of E granulosus involves two hosts.
• The adult worm of the parasite resides in the proximal small intestine of the definitive host such as dog, attached by hooklets to the bowel mucosa.
• Eggs are released into the intestine of the definitive host are passed in the feces
• Sheep are the most common intermediate hosts and ingest these eggs while grazing on contaminated soil. The egg loses its outer layer when it is digested in the duodenum. The hexacanth embryo or oncosphere that is liberated passes through the intestinal wall into the portal circulation and reaches the liver where it develops into a cyst within the liver. When the definitive host eats the meat of the intermediate host, the life cycle of the parasite continues in this manner.
• Humans become accidental intermediate hosts and the lifecycle of the parasite cannot continue further
• Transmission to humans occurs by close contact with a definitive host (usually a domesticated dog) or eating contaminated water or vegetables. Once in the human liver, cysts enlarge to 1 cm during the first 6 months and 2–3 cm yearly, depending on host immunity. The right lobe of the liver is the most frequently involved site

STRUCTURE OF HYDATID CYST
• The hydatid cyst has three layers – the outer protective pericyst, the middle laminated membrane, which is acellular and the inner germinal layer, where the scolices (the larval stage of the parasite) and the laminated membrane are produced.
• The laminated membrane and the germinal layer together make up the true wall of the cyst, usually referred to as the endocyst
• Daughter vesicles (brood capsules) are small spheres composed of protoscolices and are formed from the germinal layer. Before becoming daughter cysts, these daughter vesicles are attached via a stalk to the germinal layer of the mother cyst.
• Cyst fluid is clear or pale yellow in color, with a neutral pH, and contains glucose, protein, sodium chloride, ions, lipids, and polysaccharides. The fluid is antigenic and may contain scolices and hooklets. When vesicles rupture within the cyst, scolices are released into the cyst fluid and form a white sediment referred to as hydatid sand

HYDATID CYST LIVER
• The most common location of hydatid cysts in humans is the liver
• Diagnosis of uncomplicated liver hydatid cyst relies on clinical suspicion.
• Liver hydatid cysts if not timely recognized and treated can enlarge in size and result in the following complications
? Fistulae formation with neighboring organs or the biliary system
? Compression of adjacent organs
? Portal vein compression
? Hemorrhage
? Secondary bacterial infection
? Invasion of abdominal wall
? Hematogenous dissemination to bone spleen, brain and other sites
? Develop daughter cysts within
? Rupture into the peritoneal cavity seeding daughter cysts
? Exophytic growth – the cysts progress exophytically beyond the limits of the liver via the liver capsule, ligaments, and peritoneum.
? May rarely die

CLINICAL FEATURES OF HYDATID CYST LIVER
• Abdominal pain
• Nausea and vomiting
• Palpable liver (hepatomegaly)
• Obstructive jaundice
• Eosinophilia (in nearly 25% of cases)

DIAGNOSIS
• Ultrasound imaging is the preferred method for the diagnosis of hydatid cyst liver. This is usually validated by computed tomography (CT) and/or magnetic resonance imaging (MRI) scans.
• Cysts can be incidentally discovered on x-ray imaging
• Specific antibodies are detected by various serological tests (eg ELISA) and can support the diagnosis.

TREATMENT
There are four modalities of treatment for cystic echinococcosis:
• Percutaneous treatment of the hydatid cysts with the PAIR (Puncture, Aspiration, Injection, Re-aspiration) technique (unilocular cysts with or without daughter cysts)
• Surgery (preferred treatment method)
• Anti-infective drug treatment (Albendazole)
• Watch and wait (if cysts are stable and not enlarging and/or are calcified)

The choice of treatment depends mainly on the ultrasound images of the cyst, following a stage-specific approach, and also on the medical facilities and human resources available.

SURGERY
• Can be open or laparoscopic. Albendazole 10 mg/kg/day for 3–6 weeks before surgery should be given to sterilize the cyst.
• Surgery can be conservative or radical. Conservative surgery aims at sterilization and drainage of cyst content, including the hydatid membrane (hydatidectomy), and partial removal of the cyst. It involves puncture of cyst and aspiration of part of the content to permit introduction of scolicidal agent followed by total aspiration.
• Radical procedures involve complete removal of the cyst with or without hepatic resection. Radical procedures carry higher intraoperative risks, with fewer postoperative complications and relapses.
• After surgery, the patient should continue albendazole for at least 6–8 weeks to eliminate any spilled hydatid fluid containing live scolices and prevent relapses.

PREVENTION OF HYDATID DISEASE
• Periodic deworming of dogs with praziquantel (at least 4 times per year), better hygiene in the slaughtering of livestock (including the proper destruction of infected offal) and public education campaigns can lower human transmission
• Vaccination of sheep with an E. granulosus recombinant antigen (EG95)
• Thorough washing and cleaning of vegetables before cooking